In certain computer-aided design and two- or three-dimensional graphics applications, the images produced are often built on a skeleton structure referred to as a “wireframe”. In CAD applications, graphics or product designers often prefer to use wireframe images because they can be rendered and manipulated on screens more readily than other types of images. FIGS. 1A and 1B illustrate two-dimensional images of a design in wireframe form.
The ultimate shape of a wireframe is determined by a set of constraints and dimensions applied to individual lines and curves to form a composite design. For example, parallel lines, perpendicular lines, circles, arcs, and lines coincident or tangent to circles can be geometric constraints for a wireframe design. Dimensions for a wireframe design can be values representing the distance between two parallel lines (e.g., linear dimension), the angle between two convergent lines (e.g., angular dimension), or the radius of a circle (e.g., radial dimension).
Existing geometric constraints for wireframe designs are limited in that they can be applied only to single geometric entities, and therefore, are sensitive to topology changes of the underlying design. For example, in the free-form surface design field, a user may desire to form a section of a two-dimensional wireframe structure by defining a curve tangent to a tangent continuous set of curves (or a curve perpendicular to a tangent continuous set of curves). However, given the existing state of the art in wireframe design, it is not possible to define a curve that is tangent (or perpendicular) to a tangent continuous chain of curves independently of the chain of curves' topology. As a result, if a geometric entity in an existing wireframe design is changed, then during a replay of the design to implement the change, the intent of the design change may not be maintained if the topology of the associated chain of curves is changed. Accordingly, a pressing need exists for a wireframe design approach that can maintain the design intent if a geometric entity is changed regardless of whether or not the topology of the associated chain of curves is changed.